Here is a screenshot of GPU-Z, take a look at GPU Core Clock and GPU Load.
I think this might have something to do with recent windows update, but I checked the update history, there's nothing relates to GPU. I didn't update my GPU driver recently.
Now I can't even watch a video in fullscreen without lag.
My GPU is HD4400, I'm running Win8.1 x64.
My computer some how managed to fix itself today, I didn't do anything, not even a reboot. Now it's always 1100 MHz when plugged in.